The present disclosure relates in general to the field of well logging, and more particularly, to techniques using one or more tilted electromagnetic transmitter-receiver pairs for improved electromagnetic measurements of subsurface formations.
Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques typically use tools equipped with sources adapted to emit energy into a subsurface formation that has been penetrated by a borehole. The emitted energy may interact with the surrounding formation to produce signals that may be detected and measured by one or more sensors. Based on the detected signal data, a profile of the formation properties (for example, resistivity as a function of wellbore depth) may be obtained.
Examples of well logging tools may include electromagnetic (“EM”) resistivity tools, such as induction and propagation tools. The EM resistivity tools may be disposed within a borehole to measure the electrical conductivity (or its inverse, resistivity) of the formation surrounding the borehole. A conventional electromagnetic resistivity tool includes at least one transmitter and two receivers, each receiver being disposed some distance away from the transmitter along the axis of the tool. Conventional transmitters and receivers include antennas formed of coils having one or more turns of insulated conductive wire wound around a support. As is understood in the art, under the principle of reciprocity, each of those antennas can operate as a transmitter and/or a receiver.
EM induction tools measure the resistivity of the formation by measuring the voltage induced in a receiver by currents flowing in the formation in response to an EM signal emitted by a transmitter. In an induction tool, a transmitter coupled to an alternating current source, such as an oscillator, produces a time-varying EM signal. The EM signal from the transmitter is transmitted into the surrounding formation, which induces a fluctuating current or “eddy current” in the formation near the transmitter. The eddy current in the formation gives rise to a time-varying EM signal that induces an electrical voltage in the receiver. If a pair of spaced-apart receivers is used, the induced voltages in the two receivers will generally have different phases and amplitudes due to, for example, geometric spreading and absorption by the surrounding formation. EM propagation tools operate in a similar fashion, but typically at higher frequencies than EM induction tools.
In many conventional EM induction and propagation logging tools, the transmitter and receiver antennas are mounted with their axes along the longitudinal axis of the tool. Thus, these tools are implemented with antennas having longitudinal or axial magnetic dipole moments. A “transverse” antenna or coil has a magnetic dipole moment that is perpendicular to the tool axis, and a “tilted” antenna has a magnetic dipole moment that is neither parallel with nor perpendicular to the tool axis.
Because the EM signals transmitted and received by an EM resistivity tool pass through the surrounding formation, measurements made by such tools can provide information regarding the electromagnetic properties of the media through which the signals pass. Information such as distance to bed boundaries, formation dip, and anisotropy may be extracted from the received signals.
In the drilling industry there is an ever-increasing need for accurate well placement. Optimal placement of the borehole in the reservoir requires directional measurements on which steering decisions can be based. A further need in hydrocarbon exploration is to identify and characterize hydrocarbon reserves. For example, the formation anisotropy can be used to identify low resistivity pay residing in thinly laminated formations.
Many recent patents disclose methods and apparatus to make directional measurements and obtain resistivity anisotropy. For logging while drilling applications, U.S. Pat. No. 5,508,616 to Sato et al. discloses an induction-type tool with two coils tilted at different directions not aligned with the tool's longitudinal axis. The tool could be adapted for possible geosteering applications. The directionality of the measurement is illustrated through a simple argument that the sensitivity function of the two tilted coils is concentrated towards the overlapping region of the sensitivity area of each coil. Through rotation of the tool, Sato et al. claims that a deep azimuthal resistivity image of the formation can be obtained to help make steering decisions. However, this patent does not provide any details as to how the azimuthal resistivity can be obtained, nor does it describe any further boundary detection/characterization techniques required for quantitative geosteering decision-making.
U.S. Pat. No. 6,181,138 to Hagiwara et al. extends Sato et al.'s single fixed directional coils into co-located triple orthogonal induction coils at the transmitter and receiver locations. No tool rotation is said to be required, since the focusing direction can be tuned to arbitrary orientation through linear combination of the orthogonal coil responses.
U.S. Pat. No. 6,297,639 to Clark et al., which is commonly owned by the assignee of the present disclosure, discloses a method and apparatus for making directional measurements using various shield designs to provide selected attenuation of EM wave energy for axial, tilted, and transverse antenna coils. This patent describes, among other things, general directional induction and propagation measurements with tilted coils and appropriate shields, along with a process for conducting borehole compensation. A combination of one transmitter and one receiver coil with at least one of them being tilted with respect to the tool axis is explicitly described by Clark et al., along with its application for bed boundary direction detection by observing azimuthal variation of the induced signal as the tool rotates. The azimuthal variation of the coupling can be used for steering wells while drilling. Other shield patents have since been granted, including U.S. Pat. No. 6,351,127 to Rosthal et al., and U.S. Pat. No. 6,566,881 to Omeragic et al., both of which are commonly owned by the assignee of the present disclosure.
U.S. Pat. No. 6,476,609 to Bittar extends an earlier anisotropy patent (U.S. Pat. No. 6,163,155, also to Bittar) to the area of geosteering application. The bedding response of up/down tilted induction and propagation apparatus is described through the difference or ratio of signals at two different orientations, but no shielding is mentioned. Nor are the effects of anisotropy or dipping considered. Also lacking in the '609 patent is a description of how to use these measurements to derive a precise distance to a formation bed boundary. The '609 patent implicitly assumes that bedding orientation is precisely known so as to calculate the up/down response. No technique, however, is disclosed to locate the precise up or down direction prior to the calculation of the up-down directional signals.
U.S. Pat. No. 6,969,994 to Minerbo et al., which is also commonly owned by the assignee of the present disclosure, discloses tool configurations and symmetrization techniques that simplify the responses of the directional measurements to the point that they become almost independent of anisotropy or dip angle. Responses to bed boundaries having different dip and anisotropy essentially overlap except near the bed boundary. Both two-coil (one transmitter and one receiver: “TR”) induction style and three-coil (one transmitter and two receivers: “TRR”) propagation-style measurements can be symmetrized to achieve this simplification. The symmetrization is done using two tilted TR pairs of the same spacing, but with the transmitter tilt angle and receiver tilt angle exchanged.
U.S. Pat. No. 6,998,844 to Omeragic et al., also assigned to the assignee of the present disclosure, discloses propagation-style directional measurements for anisotropy determination in near-vertical wells with borehole compensation. Inversion techniques are also used to obtain the anisotropic formation property.
Further, commonly assigned U.S. Pat. No. 7,202,670 to Omeragic et al. discloses a method of extracting and analyzing the azimuthal dependence of directional logging measurements, using measurements taken at all azimuthal angles, to characterize the earth formation and for steering bottomhole drilling assemblies with improved accuracy. It teaches how to determine the bedding azimuth from the directional measurements, and generating measurements that can be used for well placement in up/down or azimuthal steering. It further teaches ways of utilizing these directional measurements in real-time to obtain bed boundary distances and to obtain accurate earth models such that geosteering decisions can be made for well placement. It also discloses a method of detecting the presence of resistivity anisotropy in formation layers adjacent near-vertical wells. In addition, it teaches a method to obtain structure dip information from directional measurements in vertical and low angle wells.
As described above, EM induction tools measure the resistivity of the formation by measuring the voltage induced in a receiver by currents flowing in the formation in response to an EM signal emitted by a transmitter. In general, the induced voltage in a receiver is a linear combination of all the electromagnetic coupling components, Vij (i,j=x,y,z), forming a 3×3 EM coupling tensor. In a planar geometry formation in which all the relevant bedding boundaries are parallel, there are only five non-zero elements in the 3×3 EM coupling matrix, referred to as Vxx, Vyy, Vzz, Vxz, and Vzx. The coordinate system is chosen such that the z-axis is along the tool axis and the y-axis is parallel to the boundary plane. Since those EM coupling components measure the characteristics of the formation, it is desirable to have an apparatus and method to determine them.
Certain difficulties may be experienced by using conventional propagation resistivity tools, where the magnetic dipole moments of transmitters and receivers are oriented essentially parallel to the tool axis. One is that the measurements are sensitive to or affected by the anisotropy when the relative dip angle is greater than 45 degrees. This can be understood easily in an extreme case of a vertical well, because in a vertical well, the induced current is generally parallel to the horizontal plane and, therefore, the response does not carry any information about the vertical resistivity assuming the formation layers are also parallel to the horizontal plane. Another difficulty is that the vertical resistivity and the relative dip angle are coupled. Therefore even in the relative high dip angle, simultaneous determination of horizontal resistivity (Rh), vertical resistivity (Rv), and the relative dip angle (θ) may not be possible for homogeneous formations. Additionally, environmental effects can break the coupling between Rh and θ.